Titration argentimetric

Potassium chromate may be used as an indicator producing a red colour with excess Ag ion. More widely applicable is the method of back titration. Excess AgNOj is added to the sample containing chloride or bromide ions. The excess AgNOj is then titrated with ammonium thiocyanate and ammonium ferrous sulphate is used as an indicator of excess SCN.  

Argentimetric titration is used in pharmacopoeial assays of sodium chloride and potassium chloride tablets, thiamine hydrochloride, mustine chloride and carbromal. 

HOOCCH, CHoCOOH NCH2CH2N HOOCCHj GHjCOOH Fig. 3.10 EDTA. 

The end-point of the reaction is detected using an indicator dye. The dye is added to the metal solution at the start of the titration, and forms a coloured complex with a small amount of the metal. The first drop of excess EDTA causes this complex to break up resulting in a colour change. 

Titration with EDTA is used in the pharmacopoeial assays of bismuthsubcarbonate, calcium acetate, calcium chloride, calcium gluconate, magnesium carbonate, magnesium hydroxide, magnesium trisilicate, bacitracin zinc, zinc chloride and zinc undecanoate. 

As the name suggests, these assays all involve silver nitrate (AgN03). This salt is the only water-soluble salt of silver, so reaction of silver nitrate with any other salt will result in the production of a precipitate. Salts such as sodium chloride (NaCl) and potassium cyanide (KCN) can be assayed in this way 

AgN03 + NaCl — AgCl(ppt) + NaN03 AgN03 + KCN — AgCN(ppt) + KN03 

The sample of salt is dissolved in water and titrated with standardised silver nitrate solution until all the silver salt has precipitated. Titrations of this type can be self-indicating, but usually an indicator is chosen that gives a coloured precipitate at the end point. In the assay of NaCl, potassium chromate is added to the solution once all the NaCl has reacted, the first drop of AgN03 in excess results in the precipitation of red silver chromate, which changes the colour of the sample to brown-red. 

Limit tests are quantitative or semi-quantitative tests used in the British Pharmacopoeia to identify and control small quantities of impurity that 

A typical example of a limit test is the test for salicylic acid in a sample of Aspirin BP. Salicylic acid is formed by hydrolysis of aspirin (or may be an impurity from the synthesis). The test involves comparing the violet colour produced when the sample is reacted with ferric chloride with that obtained from a standard salicylic acid solution. 

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Argentimetric titrations Compleximetric titrations Redox titrations... 

Precipitation is the term used to describe the process whereby a substance leaves solution rapidly, forming either a crystalline solid or amorphous solid (the precipitate). In the case of a precipitation titration, this process occurs when the analyte forms a precipitate with the titrant. The most common types of precipitation titrations use silver nitrate as the titrant. They are often referred to as argentimetric titrations. 

In all argentimetric titrations strong light (including daylight) should be avoided as it can lead to decomposition of the silver salts. 

In variants based on the determination of alcoholyzable halogen, chlorine and bromine have been determined69 by solvolysing the sample in a sealed tube with sodium butoxide. The resulting chloride or bromide was determined by argentimetric titration, with visual, or preferably potentiometric, indication of the end-point. 

These are titrations in which the analyte and titrant react to form a precipitate. The only common titrant used is silver nitrate (argentimetric titrations), and its use is mainly restricted to the determination of... 

The other, very popular argentimetric titration the Volhard method uses different approach. An excess of the silver chloride forming silver nitrate reagent is added to the sample and after the reaction... 

Different electrochemical methods have been worked out to indicate the endpoint of argentimetric titrations. They can be used for the analysis of chloride as well as for the other halides. Their application is especially advantageous in mechanized or automated titrators. 

Usually an indifferent platinum cathode is used in argentimetric titrations, with elec-trolytically generated silver ion reagent. It is often placed in a separated half cell. 

Iodide ions can be determined quite well with argentimetric titrations. The Volhard method, electrometric endpoint indications, and adsorption indicators work well. The Mohr endpoint indication, however, does not give good results because of the adsorption of the chromate on the silver iodide precipitate. The presence of chloride and bromide ions disturbs the argentimeric iodide determination. 

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